A coherent optical data transfer scheme of which commercial introduction began from around 2010 has now become a key technology supporting long-distance optical communication. In recent years, the coherent optical data transfer scheme has become more and more important. For example, application to a metro access network has been examined. Initially, a coherent optical data transfer scheme was realized by performing polarized-wave multiplexing Quadrature Phase Shift Keying (QPSK) modulation on an optical channel with a 100 Gbit/s capacity. Here, the polarized-wave multiplexing wave refers to a multiplexing scheme for allocating individual data to each of two orthogonal X-polarized and Y-polarized wave components included in an optical signal. For example, FIG. 14 is a diagram in which an example of a transmission system using an optical transmitter and an optical receiver that perform coherent optical data transmission shown in FIG. 1 of Non-Patent Document 1 is cited. The transmission system modulates each of the X-polarized wave and the Y-polarized wave with different 50 Gbit/s QPSK (4-level phase modulation) codes, performs polarized-wave multiplexing, and performs long-distance transmission with a polarized-wave multiplexing QPSK signal of 100 Gbit/s per wavelength.
For additional reduction of costs with respect to capacity, realization of a large-capacity optical channel of 200 Gbit/s or more with a coherent optical data transmission system using multi-level modulation such as 16QAM (Quadrature Amplitude Modulation) has been attempted. A problem apparent includes constellation distortion when multi-level modulation such as such polarized-wave multiplexing QPSK, 16QAM, and 64QAM is used. A multi-level modulation signal is handled as an electric signal of four lanes in an electrical stage. That is, on the transmission side, a signal is generated as an electric signal of four lanes and converted into a multi-level modulated optical signal by an optical modulator.
As the optical modulator, for example, a Mach-Zehnder interferometer type modulator is applied. In such an optical modulator, there is imperfection caused by an error of a bias voltage or a finite extinction ratio of the interferometer, and constellation distortion is caused by such imperfection. When constellation distortion occurs, transmitted information cannot be correctly decoded, and an increase in a bit error rate or the like occurs. Here, the constellation is also called a signal space diagram and represents data signal points resulting from digital modulation in a two-dimensional complex plane (see, for example, “constellation” shown in FIG. 14, or FIG. 2 of Non-Patent Document 2).
QPSK is a four-level phase modulation and can be regarded as one that independently performs two-level amplitude modulation on an in-phase component and a quadrature phase component. The constellation of QPSK is a form in which points are arranged on the same circumference and are 90° apart from each other. On the other hand, 16QAM and 64QAM are modulation schemes having a constellation consisting of 16 points and 64 points, respectively. In the constellation of 16QAM and 64QAM, 16 points and 64 points are generally arranged in a square form in a signal space. 16QAM can be regarded as performing amplitude modulation with 4 independent levels on each of the in-phase component and the quadrature phase component. 64QAM can be regarded as performing amplitude modulation with 8 independent levels on each of the in-phase component and the quadrature phase component.
A direct current (DC) offset is one cause of constellation distortion. Generally, a bias voltage is applied to an optical modulator so that an optical output becomes a null point. However, in a case where the bias voltage is shifted from the null point, DC offset occurs. Further, in a Mach-Zehnder interferometer constituting an optical modulator, it is ideal that an extinction ratio (on/off ratio) is infinite, that is, the optical output is completely zero at the time of being off. However, in a case where the optical output is not completely zero at the time of being off, the extinction ratio is not infinite and DC offset occurs. In the optical signal, since the DC offset appears in the form of a residual carrier, the DC offset can be confirmed by observing a spectrum of the optical signal.
The DC offset and the residual carrier due to the DC offset occur in a direct detection scheme (for example, a scheme of directly detecting the intensity of an on and off signal of 1010 with an optical reception element, also referred to as intensity modulation direct detection, or the like) rather than a coherent detection scheme that uses a local oscillation laser. In the direct detection scheme, since a residual carrier appears as a DC offset in an electrical stage on the reception side again, the residual carrier can be easily removed in an analog DC block circuit using a capacitor or the like. On the other hand, in the coherent detection scheme, when a frequency of a transmission laser and a frequency of a local oscillation laser on the reception side do not exactly coincide, the residual carrier cannot be removed in a DC block circuit without converting the residual carrier into a DC in an electrical stage on the reception side.
Further, in-phase/quadrature (IQ) crosstalk is known as constellation distortion. IQ crosstalk occurs when a phase difference between an in-phase component and a quadrature phase component is not exactly 90° due to a bias voltage error of an optical modulator.
To cope with the constellation distortion problem, a technology for measuring characteristics of an optical modulator applied to an optical transmission device in advance and compensating for the characteristics of the optical modulator using a digital signal processing device on the optical transmission device side has been disclosed (for example, see Non-Patent Document 2).